Reduced cardiac stroke volume (SV), a frequent consequence of peripheral pooling of blood, limits cardiac output in conditions of heat strain and is important in the pathogenesis of heat disorders. We are studying experimentally the reflex responses of the peripheral veins, and the circulatory and thermoregulatory consequence of peripheral venomotor activity. Forearm volume is measured with a Whitney strain gauge, and forearm venous volume (FVV) is taken as the difference between volume of the forearm with the veins congested to a standard pressure, and volume with the veins drained by gravity. 1) we will study venomotor responses during exercise and in conditions of altered cardiac filling pressure (assessed by measuring SV or central venous pressure (CVP)) produced by changing plasma volume and by peripheral pooling of blood. Peripheral pooling will be anipulated with thermal stress, and with different postures and lower body suction (LBS). We will measure esophageal and mean skin temperature (Tes and Tsk) during all experiments, so that venomotor responses during various conditions of exercise intensity and cardiac filling may be separated into thermal and non-thermal components.. 2) By comparing CVP or SV between different conditions (while controlling plasma volume) we will assess the contribution of peripheral vascular responses to peripheral pooling of blood. We will also compare control of calf venous volume to control of FVV, and compare measurements of forearm blood flow (ABF) to regional measurements of tissue thermal conductance, to test how well forearm vascular responses represent peripheral vascular responses elsewhere in the body. 3) We will study the role of the superficial veins in thermoregulation by comparing thermal control of FVV to that of forearm blood flow and chest sweating, and by determining the relation of FVV to forearm thermal conductance over a range of conditions in which FVV responds reflexly to changes in Tex of Tsk but ABF does not. 4) We will investigate effects of exercise training and heat acclimation on venomotor control and on regional conductances, and will relate changes in these responses to changes in plasma volume and SV during exercise, to see whether these peripheral vascular responses participate in the improved circulatory function in the heat which follows exercise training and heat acclimation.